let x1: undefined = undefined
let y1: null = null
// let z: string = undefined

function doSomething(x: string | null) {
    if (x === null) {
        // todo
    } else {
        console.log('Hello, ' + x.toUpperCase())
    }
}

function liveDangerously(x?: number | null) {
    console.log(x!.toFixed())
}

